Broadly speaking, adhesives used in packaging can be classified into four categories: water based, solvent based, pressure sensitive and hot melt adhesives. Of these four, currently, the water based are used most extensively. Usually the water based adhesives are based on emulsion polymers and are applied to porous cellulosic substrates. Energy from the outside, in some fashion, is applied to the system to evaporate the water in order that a strong bond may be formed. Beside this energy requirement for the formation of the bond, there is another complication with the use of water based adhesive. For a uniform coating, a good uniform wetting of the substrate surface is desired, which is not easily achieved. For the solvent based types, the requisite evaporation of the solvent introduces the solvent into the environment which both requires energy and can be deleterious to the surrounding environment.
The key requirements for pressure sensitive adhesives are that they should have good cohesive and tackifying properties at ambient conditions. These adhesives should also have good flow characteristics if they are to be used in the bulk state, otherwise they should possess sufficient consistency when dissolved in suitable solvents so that they can be coated or sprayed on the surfaces to be bonded. Usually these adhesives are prepared from a mixture of resin, polymer and a plasticizer (when required to soften the adhesive and enhance the aggressive tack).
Certain commerical block copolymers such as Shell's Kraton.RTM. and Phillip's Solprene.RTM. are widely used as the polymers in pressure sensitive adhesives. These adhesives prepared from blends incorporating these polymers have very good adhesive and strength properties at room temperature and can be processed by conventional melt coating and extrusion techniques because of their good flow characteristics. Because of this excellent combination of properties exhibited by the ABA type of block copolymer when B represents a polydiene or a polyolefin block and A represents a polystyrene block, the use of Kratons.RTM. for various pressure sensitive adhesive applications is growing in the industry.
However, these referenced block copolymers when used in combination with tackifying resins for pressure sensitive adhesive blends, have a serious drawback when blended with a major proportion of resin. Under this condition in order to realize the tackifying properties of the petroleum resin component of the blend, it is necessary to incorporate substantial levels of a plasticizer, usually a naphthenic oil whereby the aggressiveness of tack and other requisite properties of these tertiary blends can be controlled. The use of plasticizers results in a number of deleterious effects on the making and using of pressure sensitive adhesive compositions including long term degradation of adhesion.
The petroleum resins generally used as tackifiers for Kraton.RTM. type block copolymers include those produced according to the teachings of U.S. Pat. No. 3,577,398 wherein the polymeric resinous composition is obtained from the polymerization of a feedstock containing a mixture of piperylene, i.e. pentadiene-1,3 and 2-methyl-2-butene in the presence of anhydrous particulate aluminum chloride and a hydrocarbon diluent. The resinous products are reported as having softening points of about 80.degree. C. to about 110.degree. C. (col. 2, line 62) although upon stream stripping the softening point are raised to a range of about 90.degree. C. to about 110.degree. C. (col. 2, lines 69-70).
Unfortunately, as indicated above, the use of a major proportion of these resins as a tackifier for Kraton D type block copolymers requires from 5 to 30 weight percent of plasticizer in the tertiary blend to provide a useful pressure sensitive adhesive composition. The plasticizer can be introduced to modify the A(polystyrene) block e.g. aromatic process oil or the B (polydiene or polyolefin) block, e.g. naphthenic process oil.
Other teachings which show petroleum resins derived from feedstocks having as a major amount the blend of piperylene and 2-methyl butenes include U.S. Pat. No. 4,173,699 (see Examples 1-5) and U.S. Pat. No. 4,358,574, both of which have examples which describe petroleum resin products which are functionally inadequate as tackifiers for said block copolymers unless used in a tertiary blend containing a plasticizer, usually up to 10 weight percent of a naphthenic oil.
U.S. Pat. No. 4,008,360 describes a process for producing a petroleum resin useful as tackifying resin in a binary adhesive based on natural rubber by adjusting the composition of the C.sub.5 -fraction feedstock so that the weight ratio of acylic diolefins to monoolefins is from 0.40 to 0.70 and the weight ratio of cyclic diolefins to monoolefins is 0.07 to 0.35 when polymerized in the presence of aluminum chloride. Comparative Example No. 3 shows that going outside the latter ratio results in a resin having a softening point of 80.degree. C. and a molecular weight ((Mn) as measured by VPO method) of 950 and of decreased utility as a tackifier since the rolling ball tack of the binary adhesive blend was adversely affected.
It is an object of this invention to provide an improved petroleum resin for tackification of pressure sensitive adhesive blends containing an ABA block coplymer; preferably of a binary blend of said resin and said copolymer.